Symbol package support (#138)

* added SourceLink and symbol package support

* updated date to 2020

src/common.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Copyright>Copyright © 2013-2019 Akka.NET Team</Copyright>
+    <Copyright>Copyright © 2013-2020 Akka.NET Team</Copyright>
     <Authors>Akka.NET Team</Authors>
     <VersionPrefix>1.3.17</VersionPrefix>
     <PackageIconUrl>http://getakka.net/images/akkalogo.png</PackageIconUrl>
@@ -22,4 +22,16 @@ Upgrades to latest SqlClient.
   <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
     <DefineConstants>$(DefineConstants);RELEASE</DefineConstants>
   </PropertyGroup>
+   <PropertyGroup>
+    <!-- Optional: Publish the repository URL in the built .nupkg (in the NuSpec <Repository> element) -->
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <!-- Optional: Embed source files that are not tracked by the source control manager in the PDB -->
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
+    <!-- Optional: Build symbol package (.snupkg) to distribute the PDB containing Source Link -->
+    <IncludeSymbols>true</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
+  </ItemGroup>
 </Project>